The overall aim of this project is to analyze the immune response to Toxoplasma gondii in order to define which cellular immune components and parasite target antigens are involved in the control of infection and its breakdown in immunocompromised hosts. Progress was made this year in the following areas: A. Demonstration of CD8+ cytolytic activity against T. gondii infected cells. CD8+ cells from vaccinated mice were shown to lyse infected or antigen pulsed macrophages in vitro. An antigenic fraction was partially purified which contains the appropriate target antigens. B. Investigation of T cell subset requirements for reactivation. Reactivation of T. gondii in chronically infected mice was shown to be due primarily to a loss in CD8+ T cell function. C. Establishment of a MAIDS - toxoplasmosis model. Infection of mice with a retrovirus mixture (MAIDS) was shown to reduce susceptibility to challenge infection and cause partial reactivation of latent infection.